From Flagstaff, we decided
to take a detour to the
Verde district: a highly mineralized area. The district is 90 miles north of Phoenix along
the edge of the Black Hills in Yavapai County, central Arizona and locally
referred to as the Jerome district. It encloses a 30 mi
2 mineralized
terrain with two major mines along the northern edge of the town of
Jerome. The two
principal mines in the district are the
United Verde and
United Verde Extension. To get to the district, drive south
on Highway 89A from
Flagstaff. You can also visit the town on your computer by
searching for
“Jerome, AZ” with
Google Earth.
On the northwestern edge of Jerome is a large,
partially reclaimed, open pit mine that is part of the United Verde mine. The rest
of the mine lies underground and inclues 81 miles of tunnels and shafts hidden
beneath the open pit,
Cleopatra Hill and the adjacent valley. A large active quarry is visible near
the northeastern edge of town that is actually the Clarkdale quarry and cement
plant, which has little to do with the base metal mines. Evidence of the old
smelter works is visible further east near the
Verde River and expressed as black
slag with some old buildings and foundations.
If you are familiar with
gossans, you should be able to trace an extensive
belt of
rusty-colored rocks on aerial photographs on
Google Earth. I followed the gossan 7 miles north of the United
Verde Mine, and another 7 miles south! Such an extensive gossan likely hides
‘blind deposits’! My experience is that for
every mineral deposit found on the surface, there likely is many more hidden at
depth in the immediate area (and who knows how many lie hidden under the
adjacent valley sediments). And like most mines reported to have been mined
out, the United Verde is far from being mined out. In fact, the US Geological
Survey reported at least twice as much unmined ore remains in place as was
mined in the past. Actually, it is extremely rare for a major
mineral deposit to be mined out as most productive mines terminate operations
because of declining economics as prices and markets fall during bad times.
 |
Jerome, Arizona, looking towards Cleopatra Hill. The hill with the ‘J’ is
Cleopatra Hill and the United Verde open pit is the mine dump seen in
distance to the right of the J between the two prominent hills. The tops of
two head frames of the United Verde Extension mine are visible in the
foreground. |
When I was a
student in geology, the Verde district
was considered as an exploration model for
‘submarinevolcanogenic massive sulfide deposits’. One geological dictionary defines massive sulfide very simply as:
“an unusually large deposit of sulfide
minerals”. But there is more to massive sulfide deposits.
Massive sulfides bring thoughts of copper and
zinc to geologists. And for myself, I also think of
precious metals. Sulfide minerals
in many of these can enclose significant amounts of gold and silver. So much
silver was found associated with these deposits at Jerome that the recovered silver
essentially paid for the mining operations. And many banded cherts (known as
exhalites) associated with
‘submarine
volcanogenic massive sulfides’ (VMS) contain anomalous gold.
VMS refers to a type of mineralization
with zones of massive (as well as disseminated) sulfide minerals that were deposited
beneath the ocean in hydrothermal vents including white and black smokers. Over
geological time, many of these VMS deposits now lie on dry land due to tectonic
forces while modern deposits continue to form under the ocean. Where found, VMS
deposits are hosted in rocks 3.55 billion years old to the present and may even
include deposits with a hundred million tons of ore rich in gold, silver,
copper, zinc, lead and other metals.
 |
White smoker sinter column. White smokers precipitate in hydrothermal
vents and often have auriferous silica. Black smokers are sulfide rich
and associated with higher geothermal temperatures (specimen from the
United Verde mine, and on display at the Douglas Museum in Jerome). |
VMS deposits have distinct rocks such as
volcaniclastics (
breccias) that represent very active submarine vents. Some of
these are so distinct that one Canadian geologist dubbed them
‘mill rock’ years ago because he noticed
there were operating mills and mines adjacent to many of these.
In addition to volcaniclastics (mill rock),
siliceous mounds or chimneys are formed by submarine fumaroles that are referred
to as black and white smokers. Other distinct rocks include layered stratiform
rock known as exhalites that formed during eruption of heterogeneous volcanic
material in sea water: the layering was produced by sea water naturally separating
minerals by specific gravity as they slowly settled in the ocean water.
The host rocks for the VMS deposits in Jerome are
1.7 billion years old and similar in age to VMS deposits discovered in Colorado
and Wyoming in 1979 and the early 1980s. You might be wondering what submarine VMS deposits are doing in the
dry deserts of Arizona and high mountains of Colorado and Wyoming? In the
geological past, these were accreted to continents and buried by tectonic
forces related to continental drift. After millions of years of erosion
following mountain uplift, some have been exposed at the surface, while many
more remain hidden and undiscovered. Those hidden at depth are referred to as ‘blind’ deposits. Still others continue
to form in the oceans, particularly along active volcanic island chains (such
as Japan) and along spreading centers such as the Mid-Atlantic Ridge.
When I was the Senior Economic Geologist and Deputy Director at the
Wyoming Geological Survey, Conoco Minerals discovered VMS deposits in 1979 in
the Sierra Madre Mountains west of
Saratoga. But few have heard of
these deposits because of the US Forest Service. Those that were found
near the old Itmay mine, not far from the famous
Ferris-Haggarty mine, were
withdrawn from public use. Other VMS deposits were soon found in Colorado and
Wyoming. The discoveries sent shock waves through the Forest Service which had little
taste for mining; and the agency hurried to withdraw the new discoveries until
all, or portions or most VMS deposits fell within or adjacent to the withdrawals
effectively locking out mining companies and prospectors from public land.
Much of this region in Colorado and Wyoming was known
as the
Grand Encampment district, which in the 19
th century, prior
to government interference, was a major base metal district. But because of the
withdrawals, no jobs were created because of Conoco’s discoveries, no valuable
metals were mined, and the US continued out-sourcing jobs and importing base
metals.
Today,
the Verde district is idle and the town of Jerome is a tourist attraction built
on the flank of Cleopatra Hill which rises more than 6,000 feet above sea
level. Some buildings in Jerome were constructed on the
Precambrian rock that forms Cleopatra Hill, while the rest of the town slopes
downward on Tertiary to Recent
fanglomerates and landslide debris. The
landslide has been a historical problem and periodically slides following
periods of high runoff. At the base of the slope, the surface flattens eastward
to the Verde River valley at Clarkdale and Cottonwood 4 to 5 miles east of
Jerome at an elevation of 3,300 feet.
Production
Jerome was a
major copper-gold-silver-zinc-lead district. According to the Arizona
Department of Mines and Mineral Resources (2007), 48 of 70 known VMS deposits found
in Arizona have produced ore - all of which are Precambrian age (1.7 to 1.8
billion years old). Production from these totaled more than 55 million tons.
Most of these deposits are steeply plunging and deformed, such as the United
Verde, which has been described as a rod-like ore body or pipe located within
an axis of a major, steeply-plunging, fold.
About 98% of the
past production in the Verde district was from the United Verde and United
Verde Extension mines. Even though mining ceased in 1953, there is evidence
that the miners left behind considerable ore. The district was considered to be
a copper district, but at today’s metal prices, the district would be
classified as a copper-gold-silver-zinc district. Considerable copper, gold, silver
with some zinc and small amounts of lead were recovered from the two principal
mines.
In 1989, the US Geological Survey indicated
records from Phelps Dodge showed production from the United Verde Mine from
1884 to 1975 amounted to more than 2,926,900,000 pounds ($10.1 billion at 2012
prices) of copper, 97,891,000 pounds ($82 million) of zinc, 459,000 pounds ($390
thousand) of lead, 49,603,000 troy ounces ($1.4 billion) of silver, and
1,354,200 troy ounces of gold. Koschmann and Bergendahl of the US Geological
Survey reported gold production was even higher at 1,571,000 ounces ($2.5
billion) over the lifetime of the district. Based on statistics from the U.S. Geological
Survey and Phelps Dodge, the United Verde produced $14.8 billion in metals at 2012
prices.
 |
| High wall of the United Verde open pit mine with head frames in foreground. Note large distinct, rusty gossan produced by oxidation of massive sulfides in the background. The massive sulfides were so rich that they actually ignited and burned underground for many years! |
History
The cupriferous gossan exposed at the surface at Jerome was found by
prehistoric tribes hundreds of years ago. In 1875, the U.S. Army rediscovered
the gossan and in the following year (1876) prospectors rushed to the district
to claim the discovery. By 1882, much of the minable land had been consolidated
by the newly formed United Verde Copper Company which began extracting
near-surface oxidized and supergene enriched copper ore rich in gold and
silver. Underground mining began in 1883, and open pit mining was initiated in
1920.
A second company, (United Verde Extension [UVX]),
was organized in 1899 to search for a suggested, down-faulted continuation of
the United Verde massive sulfide. UVX began searching southwest and east of the
United Verde property. Their efforts were fruitless until they were on the
verge of bankruptcy in 1914, when a drift driven from a shaft east of the
United Verde mine intersected bonanza grade ore rich in chalcocite on the 1,200
level. In 1916, another ‘blind’ deposit was found. The ore was so rich the
company ended the year with a 74% profit after expenses. Later geological
studies showed the UVX discoveries were not part of an off-set of the United
Verde ore body, but instead completely separate, ‘blind’ ore bodies.
The UVX mine
operated on a large scale until 1938 when much of its high-grade was mined out
and operations ceased. The nearby United Verde mine continued mining. After
1931, much of their activity focused on open-pit mining. Depletion of ‘high-grade’
reserves finally forced the mine to close in 1953. This resulted in many people
reporting that the deposit was mined out. However, the operations left behind a
giant, low-grade, ore deposit and who knows how many ‘blind’ deposits.
Geology
.JPG) |
| Banded Chert from the United Verde mine |
The VMS erupted
as hot, sulfide-rich material into cool water resulting in precipitation on the
ocean floor 1.7 billion years ago. The Verde VMS lies at the top of a submarine
rhyolite dome and flow breccia known as the Cleopatra Member of the Deception
Rhyolite. The ore is zoned and capped by chert and siliceous massive sulfide
that grades down into pyrite-rich massive sulfide, zinc-rich massive sulfide,
copper-zinc-rich massive sulfide, chloritic stringer ore, and chloritized
quartz porphyry ore. Gold is found throughout but is in greater concentrations at
the stratigraphic top (siliceous massive sulfide and chert ore).
Gold is in greater concentrations in chert. For instance, on the
4500-foot level, the black schist yielded 0.002 opt Au and 0.71 opt Ag. In
chert near the surface, the ore yielded values as high as 0.12 opt Au and 4 opt
Ag (60 times more gold). On the 700-foot level, the massive sulfide averaged 10
times more gold than the black schist. A positive correlation of gold with
higher zinc content was also recognized by the US Geological Survey. This is
important as much zinc ore was not mined.
Sulfides are
exposed at the surface down to the 4500-foot mine level, but in the geological
past, the ore also extended upwards to the base of the Cambrian Tapeats
Sandstone indicating at least 350 feet of vertical column of massive sulfide was
removed by erosion. With this much erosion, drainages down-slope from the
massive sulfide should contain detrital gold, but I could not find any report
suggesting that gold had been detected in the nearby drainages. Based on the
mode of occurrence, the gold would likely be very fine grained.
Ore minerals in
the massive sulfide in decreasing abundance included pyrite, sphalerite,
chalcopyrite, bornite, arsenopyrite, galena, tennanite and gold (electrum).
Much of the electrum is thought to be present as microscopic inclusions in
sulfide minerals. Silver is present in the electrum and in late-stage tennanite
(Cu12As4S13) in quartz carbonate
veins and in carbonate rich massive sulfide ore. The tennanite also contains
gold as microscopic inclusions.
The Verde was
the largest known volcanogenic massive sulfide deposit in the US with 33
million tons of mined ore, and 50 to 70 million tons of low-grade ore left in
place. It is also considered to be one of the largest in North America. The
massive sulfide was steeply-dipping, irregular- to cylindrical-shaped ore body
approximately 700 to 800 feet in diameter that extended 2,400 feet deep. The United
Verde mine reached a depth of 3,515 feet and included eight shafts. According
to the US Geological Survey, proximal ore recovered from the mine averaged
4.77% Cu, 0.046 opt Au, and 1.65 opt Ag. Zinc and lead were not recovered
during much of the operations. Another blind ore body was found in the lower
levels of the mine 500 feet northwest of the roots of the main deposit.
Anomalous gold was
detected in the pyritic and sphalertic massive sulfides and the chert rich ore:
chloritic host rocks are poor in gold. The zinc ore contained as much gold as
the copper ore. And because only the high-grade copper-rich portion of the
massive sulfide lens was mined, a very large tonnage of low-grade ore remains
in place and it was estimated that at least 75% of the mineralized deposit
remains unmined because it was too low-grade to mine (<2% Cu) under
historical economics. The estimates suggest about 115,000,000 tons of low-grade
massive sulfide ore and 38,000,000 tons of mineralized black schist remain in
the mine with grades of 0.5 to 1% Cu, 2 to 4% Zn, 0.01 to 0.015 opt Au and 0.5
to 1.0 opt Ag. This zinc-rich deposit is estimated to have 400,000 troy ounces
of gold.
At the United Verde Extension Mine, mineralization
occurred in an elliptical ore body hosted by the Deception Rhyolite and
Grapevine Gulch Formation. The main chalcocite ore body was found on the 1400-level
and was 269 feet wide and 440 feet long on this level.
 |
You may not appreciate this exhibit at the Audrey Head-frame Park in Jerome if you have.
a fear of heights. Sunlight is projected down the 1900-foot deep Audrey shaft so visitors can
see the depth of these mines. When I took this photo, I was standing on the plexiglass
covering the deep shaft. |
The UVX mine produced 3,879,000 tons of ore that averaged
10.23% Cu, 0.039 opt Au, and 1.71 opt Ag. Ore minerals included chalcocite,
cuprite, native copper, malachite, chrysocolla and azurite hosted by felsic
fragmental rocks, massive rhyolite and quartz porphyry – all units of the
Deception Rhyolite.
Part of the UVX operation was developed by the
Haynes Shaft due west the United Verde open pit. This deposit was another blind
ore body found at 2,500 feet below the surface. The UVX decided to explore the
ground under the Haynes shaft by driving a tunnel at the 3,000 level. A drift
was started in 1930 and reached the Haynes massive sulfide pipe in 1931.
The Haynes proximal
pipe was a steeply-plunging massive sulfide. It extended a short distance above
the 2700-level downward to an undetermined point between the 3450 and 3700
levels of the United Verde Mine. The Haynes haft was sunk to 700 feet between
1907 and 1911; subsequently deepened to 1,200 feet, with 1,700 feet of drifts
and crosscuts on the 700 level and 700 feet on the 1200 level.
.JPG)
